In recent years, optical filters which sufficiently transmit light in a visible wavelength region but shield light in a near-infrared wavelength region have been used for various applications.
For example, a solid-state image sensing device (such as CCD or CMOS) is used in an imaging device such as a digital still camera and a digital video camera. In order to make sensitivity of the solid-state image sensing device close to visibility of a human being, an optical filter is disposed between an imaging lens and the solid-state image sensing device.
Among them, as an optical filter for an imaging device, a glass filter has been known in which a CuO or the like is added to a fluorophosphate-based glass or a phosphate-based glass so as to enable selective absorption of light in the near-infrared wavelength region. However, in addition to being expensive, a glass filter of light-absorbing type may not sufficiently exhibit a function based on components of the glass when being made thin, and there has been a problem that it cannot sufficiently respond to requirements of size reduction and thickness reduction of an imaging device of late years.
Accordingly, in order to solve the above-described problems, there has been developed an optical filter in which, for example, a silicon oxide (SiO2) layer and a titanium oxide (TiO2) layer are stacked alternately on a substrate as a reflection layer, and the reflection layer reflecting light in the near-infrared wavelength region by interference of light and a resin layer containing a dye absorbing near infrared rays are stacked (see, for example, Patent Reference 1 (JP-A 2008-051985)).
However, in these conventional optical filters for imaging devices, performance for shielding light of wavelengths in the near-infrared region and a transmitting property of wavelength band (630 nm to 700 nm) required for imaging a dark part more brightly are insufficient. Moreover, since there is also a constraint on layer formation such that the function of the solid-state image sensing device should not be hindered, currently an optical filter having a sufficient near-infrared cut filter function has not been obtained.
In this respect, in Patent Reference 2 (JP-A 2012-008532), use of a squarylium-based dye as a near-infrared absorbing dye is described. The dye described in Patent Reference 2, although having high transmittance of 630 nm to 700 nm as compared to conventional near-infrared absorbing dye, this transmittance is not sufficiently high, and solubility to the resin is also low. Thus, there has been a problem that the near-infrared absorbing layer cannot be made sufficiently thin.